Previously known internal combustion engines, particularly reciprocating piston type internal combustion engines, are well known to include at least one valved intake port providing fluid communication between each cylinder and an intake passage, as well as at least one valved exhaust port providing fluid communication between each cylinder and the exhaust passage. Since pressure variations at the port may lag behind the time at which the piston reaches a top dead center (TDC) or bottom dead center (BDC) position when the port should theoretically open or close, valve opening and closing may be delayed a sector of crankshaft rotation occupying a finite crank angle duration. This valve overlap permits the intake valve to open the intake port while the exhaust valve also opens the exhaust port. This overlap of the valve openings has been found useful to increase the horsepower available from the internal combustion engine.
Recent regulations to control the emission of unburnt hydrocarbons and other emissions have prompted increased scrutiny of the fluid flowing through the ports and the cylinder. Some studies have indicated that a high concentration of hydrocarbon emissions during the final moments at which the exhaust port is open before closing. One of the causes believed to contribute to high concentration of hydrocarbons released during the final moments of exhaustion from the cylinder is the de-absorption of hydrocarbons from the oil and fuel coating on the cylinder walls which can absorb the hydrocarbons as they are introduced to the cylinder. In addition, the high pressures created in the cylinder, particularly due to combustion, can also increase the absorption of hydrocarbons in the lubrication boundary layer of the cylinder. As a result, these hydrocarbons may not be released until the end of the exhaust stroke when pressure has been substantially reduced in the cylinder and hydrocarbons can be de-absorbed to the exhaust gases. In addition, the rising of the piston in the cylinder toward the exhaust opening can produce a shovel effect in which the piston rings scrape the cylinder walls during the exhaust stroke to release hydrocarbons which are then pushed to the port and released at the end of the stroke. As a result, a benefit of increased horsepower by overlapping the opening of the intake and exhaust ports may be accompanied by an increase of late unburnt hydrocarbon emissions as the intake pressure forces additional fluid through the exhaust port during the valve overlap.